1. Field of the Invention
This invention relates to a passenger weight measuring apparatus for detecting the weight of a passenger sitting in a seat of a vehicle.
2. Description of the Related Art
Hitherto, an attempt has been made to control an air bag system or an emergency restraint system for a seat belt installed in a vehicle in response to the physique of a passenger. As the attempt, for example, the weight of the passenger is measured with a weight measuring apparatus placed in a seat and the expansion gas amount and the expansion speed of the air bag are adjusted or the pretension of the seat belt is adjusted in response to the measured weight.
As the weight measuring apparatus, for example, JP-A-11-337393 discloses an apparatus. In the weight measuring apparatus, load sensors 81a, 81b, 82a, and 82b placed back and forth in the left of a seat and load sensors 83a, 83b, 84a, and 84b placed back and forth in the right of the seat output voltage signals responsive to the loads added to the sensors in response to the weight of the passenger sitting in the seat, as shown in FIG. 8. Each of the load sensors 81a, 81b to 84a, and 84b includes a resistor strain gauge put on a sensor plate (skew member) bent upon application of a load, and outputs a voltage signal changing in response to the resistance value change amount accompanying the bend deformation of the sensor plate responsive to the load. The difference value between the voltage signal output by the left load sensors 81a, 81b, 82a, and 82b and reference voltage E/2 is found by a differential amplifier 85, and the difference value between the voltage signal output by the right load sensors 83a, 83b, 84a, and 84b and the reference voltage E/2 is found by a differential amplifier 86. Further, the voltage difference between both the difference values is found by an adder-subtracter 87. A weight calculation section 88 finds the weight of the passenger from the voltage difference.
However, the sensor plate on which the resistor strain gauge is placed is designed for the strength capable of measuring the weight of the passenger with good sensitivity and thus if the vehicle collides and a large shock is applied, an excessive load is applied to the sensor plate and the sensor plate may becomes abnormally deformed. In this case, the sensitivity of the load sensor deviates and it may become impossible to measure the weight of the passenger with good accuracy.
Thus, applying such a shock abnormally deforming the sensor plate to the vehicle is detected and when the shock is detected, the driver or the passenger needs to be informed that it is made impossible to normally control the air bag, etc., based on the weight of the passenger or the fact needs to be stored in a storage unit for inspection.
To solve such a problem, it is considered that a shock detector for detecting applying such a shock abnormally deforming the sensor plate to the vehicle is provided in addition to the weight measuring apparatus.
As such a shock detector, it is possible to use an air bag start control unit proposed in JP-A-5-147491, for example. When a shock such that the control unit operates for starting the air bag is applied to the vehicle, it is determined that the sensor plates of the load sensors become abnormally deformed.
However, such a unit operates in response to a considerably large shock detected for starting the air bag. Thus, when the shock applied to the vehicle is weak as in a collision at low speed and the weight of the passenger is heavy, a state in which abnormal deformation of the sensor plate cannot be detected occurs although an excessive load is imposed on the sensor plate.